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At least 739 records · Page 41Linked to original sources
PubMed2002

Population dynamics of sealworm, Pseudoterranova decipienssensu lato, in sculpins, Myoxocephalus scorpius, from two areas in Norway between 1990 and 1996.

The abundance of sealworms (Pseudoterranova decipienss.l.) in sculpins (Myoxocephalus scorpius) was studied at Torbjørnskjaer, southeast Norway, and Vega, northern Norway, in samples from 1990 to 1996. The harbour seal ( Phoca vitulina) population at Torbjørnskjaer declined by 70% during the phocine distemper virus epizootic in 1988, and has increased during the 1990s. The abundance of sealworms in sculpins was relatively stable from 1992 to 1996 and any effect of the reduction in the number of harbour seals on the abundance of sealworms has been short-term. When correcting for size differences, young sculpins (2-4 years) from Torbjørnskjaer had higher abundances than those from Vega, while in old sculpins (5-7 years) the abundances seemed similar and stabilised. This happened at a younger age among the more heavily infected sculpins at Torbjørnskjaer than at Vega, and we interpret this as density dependence in the fish host.

Age Distribution↗
PubMed2001

A new geographical gradient in vole population dynamics.

A new geographical gradient in the dynamics of small rodents is demonstrated by analysing 29 time series of density indices of the common vole (Microtus arvalis) from Poland, the Czech Republic and the Slovak Republic. This gradient extends from more stable northerly populations in coastal Poland to more variable and cyclic populations in the southernmost parts of the Slovak Republic, and is hence a reversal of the Fennoscandian gradient. All studied variables (such as mean density, cycle amplitude, density variability and the coefficients in a second-order autoregressive model) exhibit consistent latitudinal variation. Possible underlying factors are discussed. In particular, we suggest that seasonality may be a key element in explaining the observed new gradient.

Animals↗
PubMed2001

Population dynamics of a scrapie outbreak.

A detailed analysis of a scrapie outbreak in a flock of Cheviot sheep is described. A total of 33 cases of 1473 sheep born to the flock were reported between 1985 and 1994. The epidemiology of scrapie can only be understood with reference to sheep demography, the population genetics of susceptibility to scrapie, pathogenesis during a long incubation period, and the rate of transmission (by both horizontal and vertical routes), all of which interact in complex ways. In recent work a mathematical model incorporating these elements was developed and successfully reproduced key features of an earlier outbreak of scrapie in this flock. Here an application of the model to the second outbreak is described. The model accurately reproduces observed allele frequencies and total numbers of susceptible animals remaining at the end of the outbreak. A major difference between the two outbreaks is the very much lower force of infection in the second outbreak. This provided additional information which suggested two ways in which our existing assumptions be refined; firstly, older animals have reduced susceptibility to scrapie and secondly, homozygous and heterozygous susceptibles have different incubation periods.

Animals↗
PubMed1996

Development of a computer model of the population dynamics of Amblyomma variegatum and simulations of eradication strategies for use in the Caribbean.

Control or eradication efforts should cover wide areas because of long-range migration potential of immature ticks attached to birds, such as cattle egrets. Careful follow-up inspections are also imperative. Simulation 1 shows how rapidly the population numbers may expand from small numbers, either introduced or missed. Simulation 4 indicates extending the treatment cycle beyond the residual effectiveness of the acaricide to save labor or funds would be false economy. The result could be to maintain the tick population indefinitely, with an accompanying increase in long-term control costs. Simulations 2 and 3 indicate that eradication is possible if response is prompt and the treatment cycle is less than the period of residual effectiveness of the acaricide used.

Animals↗
PubMed2004

Simulation of the population dynamics and social structure of the Virunga mountain gorillas.

An agent-based model was developed to simulate the growth rate, age structure, and social system of the endangered mountain gorillas (Gorilla beringei beringei) in the Virunga Volcanoes region. The model was used to compare two types of data: 1) estimates of the overall population size, age structure, and social structure, as measured by six censuses of the entire region that were conducted in 1971-2000; and 2) information about birth rates, mortality rates, dispersal patterns, and other life history events, as measured from three to five habituated research groups since 1967. On the basis of the research-group data, the "base simulation" predicted a higher growth rate than that observed from the census data (3% vs. 1%). This was as expected, because the research groups have indeed grown faster than the overall population. Additional simulations suggested that the research groups primarily have a lower mortality rate, rather than higher birth rates, compared to the overall population. Predictions from the base simulation generally fell within the range of census values for the average group size, the percentage of multimale groups, and the distribution of females among groups. However, other discrepancies predicted from the research-group data were a higher percentage of adult males than observed, an overestimation of the number of multimale groups with more than two silverbacks, and an overestimated number of groups with only two or three members. Possible causes for such discrepancies include inaccuracies in the census techniques used, and/or limitations with the long-term demographic data set obtained from only a few research groups of a long-lived species. In particular, estimates of mortality and male dispersal obtained from the research groups may not be representative of the entire population. Our final simulation addressed these discrepancies, and provided a better basis for further studies on the complex relationships among individual life history events, group composition, population age structure, and growth rate patterns.

Age Factors↗
PubMed2005

Phylogeography, population dynamics, and molecular evolution of European bat lyssaviruses.

European bat lyssaviruses types 1 and 2 (EBLV-1 and EBLV-2) are widespread in Europe, although little is known of their evolutionary history. We undertook a comprehensive sequence analysis to infer the selection pressures, rates of nucleotide substitution, age of genetic diversity, geographical origin, and population growth rates of EBLV-1. Our study encompassed data from 12 countries collected over a time span of 35 years and focused on the glycoprotein (G) and nucleoprotein (N) genes. We show that although the two subtypes of EBLV-1--EBLV-1a and EBLV-1b--have both grown at a low exponential rate since their introduction into Europe, they have differing population structures and dispersal patterns. Furthermore, there were strong constraints against amino acid change in both EBLV-1 and EBLV-2, as reflected in a low ratio of nonsynonymous to synonymous substitutions per site, particularly in EBLV-1b. Our inferred rate of nucleotide substitution in EBLV-1, approximately 5 x 10(-5) substitutions per site per year, was also one of the lowest recorded for RNA viruses and implied that the current genetic diversity in the virus arose 500 to 750 years ago. We propose that the slow evolution of EBLVs reflects their distinctive epidemiology in bats, where they occupy a relatively stable fitness peak.

Animals↗
PubMed1999

Population dynamics of scrapie in a sheep flock.

A detailed analysis of an outbreak of natural scrapie in a flock of Cheviot sheep is described. A total of 137 cases was reported over 13 years among 1307 sheep born into the flock. The epidemiology of scrapie can only be understood with reference to sheep demography, the population genetics of susceptibility to scrapie, pathogenesis during a long incubation period, and the rate of transmission (by both vertical and horizontal routes), all of which interact in complex ways. A mathematical model incorporating these features is described, parameter values and model inputs are derived from available information from the flock and from independent sources, and model outputs are compared with the field data. The model is able to reproduce key features of the outbreak, including its long duration and the ages of cases. The analysis supports earlier work suggesting that many infected sheep do not survive to show clinical signs, that most cases arise through horizontal transmission, and that there is strong selection against susceptible genotypes. However, important aspects of scrapie epidemiology remain poorly understood, including the possible role of carrier genotypes and of an environmental reservoir of infectivity, and the mechanisms maintaining alleles giving susceptibility to scrapie in the sheep population.

Alleles↗